In recent years hard contact lens materials having improved oxygen permeability have been developed. Certain such materials are set forth in U.S. Pat. No. 3,808,178 which describes contact lenses fabricated from a copolymer of a polysiloxanyl acrylic ester and an alkyl acrylic ester. Other such hard contact lens materials have been developed. It is sometimes difficult to obtain good dimensional stability in contact lenses made from siloxanyl alkyl ester vinyl monomers.
Dimensional stability is an important property of hard contact lenses and affects both accurate vision correction and wearer comfort. It is known that changes in the dimensions of hard lenses can occur rapidly shortly after cutting and finishing or over a prolonged period of time. Such changes can be of various types. Changes to the base curve or outer curve of a contact lens which changes are uniform are known as "steepening" or "flattening" depending upon the direction of change. A non-uniform change in lens dimension is termed "warpage".
Dimensional changes of the type noted above can result from one or more factors which include the relief of internal stress or strain incurred during manufacture or the takeup or loss of material within the lens. The prior art has developed processes for handling internal stress and strain problems as by the use of careful annealing of lens blanks.
Dimensional stability difficulties have been encountered with gas permeable contact lenses prepared from co or higher polymers containing a siloxanyl alkyl ester vinyl monomer. These difficulties may result from the differences in the reactivity ratios between various monomers employed in the materials and the chain transfer reactions during polymerization. Contact lenses produced from such materials can contain significant portions of unreacted monomer or monomers. This can lead to dimensional instability when these monomers leave the material. The monomers can leave the lenses shortly after making the lenses or over a long period of time. When the monomers leach out during use, this can have toxicological consequences.